Silencer shells (mufflers) and other types of housings such as those used in the automotive industry are formed by wrapping a relatively thin wall sheet around a mandrel that has the same shape as the desired silencer (muffler) shell, then mechanically locking the two sheet edges of the sheet together using a process called lockseaming. The sheet is usually made of galvanized or otherwise corrosion-resistant steel, as is known in the art.
The lockseaming process of the prior art involves the use, first, of gathering rollers, and second, of lockseaming rollers. In the prior art (described further below), after the sheet has been wrapped onto the mandrel, the gathering rollers are passed over the sheet on the mandrel, i.e., from a leading side of the sheet to a trailing side thereof. Subsequently, the lockseaming rollers are passed over the sheet on the mandrel in the same direction, i.e., from the leading side to the trailing side. As will be described, however, the prior art process results in drag on the sheet. Such drag forces sheet material away from the vertical center plane of the mandrel, especially toward the trailing side, and tends to cause edge portions of the sheet towards the trailing side of the sheet to be further apart than desired. As a result, the lockseam in the portion of the lockseamed shell towards the trailing side tends to be of lower quality.
The prior art technique for forming thin wall sheet into an automotive muffler shell 62 will now be described, with reference to FIGS. 1, 2, 3A, 3B, 4A, 4B, 5A, 5B, 6, 7A, 7B, 7C, 7D, 7E, 8 and 9. (As will be described, the remainder of the drawings illustrate the present invention.) The forming technique according to the prior art includes, first, providing a mandrel 1 that is shaped so as to form a muffler shell in accordance with a predetermined design (FIG. 1). As is known, the muffler shell may have any of a variety of shapes, i.e., it may be round, oval, or irregular in cross-section.
In the prior art, a thin wall sheet 2 is placed under the mandrel 1 and above forming rollers 3, 4 and a pressure bar 12 (FIG. 1). The rollers 3, 4 are mounted on arms 5, 6. The arms 5, 6 are mounted for rotation around centers 7, 8 which are mounted on a platform 11. Springs 9, 10 urge the arms 5, 6 against the mandrel 1, i.e., in the directions indicated by arrows “A” and “B” (FIG. 2).
The pressure bar 12 preferably is mounted on a pneumatic cylinder 13. As shown in FIG. 1, at the beginning of the prior art process, the pressure bar 12 is spaced apart from the mandrel 1 sufficiently to permit the sheet 2 to be positioned between the pressure bar 12 and the mandrel 1. After the sheet 2 has been positioned as shown in FIG. 1, the pressure bar 12 and the platform 11 move relative to the mandrel 1 in the direction indicated by arrow “C” in FIG. 1 to result in the sheet 2 being pressed tightly against the mandrel 1 by the pressure bar 12 at a point where a lowest part of the mandrel surface is intersected by the vertical center plane 58 of the mandrel, as shown in FIG. 2.
Preferably, the platform 11 is moved up and down by one or more hydraulic cylinders 13, however, the movement may be effected by any suitable means. As the platform 11 moves up (i.e., in the direction indicated by arrow “C”), the forming rollers 3, 4 along with the arms 5, 6 are rotated around the centers 7 and 8 and the springs 9, 10 urge the rollers 3, 4 against the sheet 2, pressing the sheet 2 against the mandrel 1, so that the sheet 2 is partially formed around the mandrel 1 as shown in FIG. 2.
As can be seen in FIGS. 2 and 3, the pressure bar 12 holds a portion of the sheet 2 firmly against the mandrel 1 so that the sheet 2 does not move in a horizontal plane relative to the mandrel 1 while the platform 11 is moving upward and the rollers 3 and 4 are forming the sheet 2 around the mandrel 1.
The prior art process and apparatus described herein disclose only one of the common methods of the prior art. Other means of forming the sheet 2 around the mandrel 1 are known to those who are skilled in the art.
Typically, the mandrel is stationary, and a roll carriage 14 (FIG. 6) is adapted to move relative to the mandrel 1. Various means for mounting and moving the prior art roll carriage 14 relative to the mandrel 1 are well known to those skilled in the art. In the prior art, the roll carriage 14 includes gathering rollers 15, 16 and vertical rollers 17 which are mounted so as to engage the sheet 2 after the sheet 2 has been generally positioned on the mandrel 1 by the forming rollers, as shown in FIG. 2.
The gathering rollers 15, 16 and the vertical rollers are allowed to travel on top of the mandrel 1 and substantially symmetrically about the center plane 58 of the mandrel 1 so that two edge portions 39, 40 (FIG. 3B) of the sheet 2 are gathered together by the gathering rollers 15, 16 and formed against the vertical roller set 17.
As the roll carriage 14 travels along the center plane 58 of the mandrel 1, the gathering rollers 15, 16 progressively push the edge portions 39, 40 closer together, as shown in FIGS. 4A, 4B.
The roll carriage 14 (FIG. 6) typically also includes lockseaming rollers 18, 19, which are mounted so that the lockseaming rollers 18, 19 engage the edge portions 39, 40 (FIGS. 5A, 5B) after the sets of gathering rollers 15, 16 and the vertical roller 17 have done so. As is well known in the art, each of the sets of rollers 15, 16, 17, 18, 19 consists of a number of rollers of different shapes which are arranged so that they progressively form the two edge portions 39, 40 into a lockseam 41 (FIG. 7A).
The ideal form of lockseam 41 is shown in FIG. 7A. As can be seen in FIG. 7A, in the ideal form of lockseam, the edge portions 39, 40 are folded over into each other to provide a secure seam, as is known.
In the prior art, the number of rollers in the sets of gathering rollers 15, 16 varies depending on a number of factors, such as design and preference. Typically, each set of gathering rollers 15, 16 includes seven to nine rollers. Similarly, the number of rollers in the set of rollers 17 varies depending on design and preference. Typically, three rollers are included in the set of rollers 17.
Also, the number of rollers in the sets of lockseaming rollers 18,19 varies depending on design and preference. Typically design includes seven to nine rollers in each of the sets of rollers 18, 19.
Many variations to the prior art arrangements described above are known to those skilled in the art. For example, the springs 9 and 10 may be replaced by hydraulic or pneumatic cylinders or any other suitable means for urging the forming rollers against the mandrel 1.
As indicated above, the roll carriage 14 (i.e., including the gathering roller sets 15, 16, the vertical roller set 17, and the lockseaming roller sets 18, 19) travels in a path substantially along the vertical center plane 58 of the mandrel 1, in which an axis of the mandrel lies. The various roller sets engage the sheet on the mandrel respectively, as the roll carriage 14 moves relative to the mandrel 1. The gathering rollers 15 progressively bring the edge portions 39, 40 together and form the edge portions 39, 40 over the vertical roller set 17. Furthermore, in ideal conditions (i.e., when the edge portions 39, 40 are sufficiently large to be formed into an acceptable lockseam (see, e.g., FIG. 7A)), the lockseaming rollers 18, 19 progressively form the two edge portions 39, 40 into the ideal lockseam 41 as shown in FIG. 7A.
However, in practice, conditions often are not ideal. The process of gathering the edge portions 39, 40 together (i.e., when the gathering rollers 15 are moved along the edge portions 39, 40 and engage the edge portions 39, 40) causes “drag” on the sheet 2, tending to cause the edge portions 39, 40 to be spaced progressively further apart. The edge portions 39, 40 tend to be spaced further apart due to drag the closer they are to the trailing side of the sheet. For illustrative purposes, FIGS. 7A-7C show the finished shell 62 with the ideal lockseam 41 (FIG. 7A) at the leading side (i.e., to the left as presented in FIG. 7C), as viewed by an observer viewing the shell from the leading side, and an inadequate lockseam 63 (FIG. 7B) at the trailing side (i.e., to the right as presented in FIG. 7C), as viewed by an observer viewing the shell from the trailing side. FIGS. 7D and 7E are end views of the shell 62 after removal thereof from the mandrel 1.
Drag is exacerbated when forming somewhat larger muffler cross-sections such as relatively large, round mufflers and asymmetric shapes, for example, as illustrated in FIG. 8.
As can be seen in FIG. 8, when the shell is larger, the forming rollers 3 and 4 are placed at somewhat greater distances from the gathering rollers 15, 16, and also from the lockseaming rollers 18, 19. Because of this, sections 42, 43 of the sheet 2 are not maintained firmly pressed against the mandrel 1 between the gathering and lockseaming rollers 15,16, 17, 18, and 19 and the forming rollers 3, 4. The drag on the muffler sheet edge portions 39, 40 forces the sheet material downwardly into the areas 42, 43. This tends to be more pronounced toward the trailing side of the sheet. Because of this, it sometimes happens that insufficient material for lockseaming is present in the edge portions 39, 40 resulting in deterioration of the lockseam 41 towards the trailing side. A typical defective lockseam (i.e., due to insufficient material in the edge portions 39, 40 toward the trailing side) is shown in FIG. 7B. Also, if the edge portions 39, 40 deviate sufficiently from design requirements, then the lockseam may not be formed at all.
As noted above, FIG. 8 shows that, where the shell to be formed is larger, the forming rollers 3, 4 are positioned, at their highest positions, relatively far from the gathering rollers 15, 16 and the lockseaming rollers 18, 19. This tends to result in drag. FIG. 9 shows the typical consequence of positioning the forming rollers 3 and 4 (at their highest positions) closer to the top of the mandrel, in order to address the problem illustrated in FIG. 8. However, and as shown in FIG. 9, positioning the forming rollers higher causes a different problem. By forming a larger section of the sheet 2 around the mandrel 1 as shown in FIG. 9, the forming rollers 3, 4 are closer to the gathering rollers 15,16 and 17 and the lockseaming rollers 18, 19. However, the sheet edge portions 39, 40 are folded over each other (FIG. 9) because of the relatively high positioning of the forming rollers 3, 4. In this situation, the vertical roller set 17 (not shown in FIG. 9) is unable to separate the two sheet edge portions 39, 40 and to provide them in the required (separated) position so that they can be engaged by the gathering rollers and, subsequently, the lockseaming rollers. Accordingly, the required lockseam cannot be formed when the prior art conditions shown in FIG. 9 are present.
Therefore, there is a need for an improved lockseaming process and apparatus.